This section introduces aspects that may facilitate better understanding of the disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
In a mobile communication system, in order to improve data rate and signal transmission/reception performance, multiple antenna techniques have been widely used. For instance, a signal transmitted from multiple antennas can be weighted in both magnitude and phase by beam-forming so as to produce spatial selectivity, that is, to produce a constructive signal power gain in a desired direction and null or attenuated interference power in other directions.
Eigenvector based beam (EBB) and Grid of beam (GOB) are conventional beam-forming approaches. In general, the EBB approach provides better performance, especially in a multipath channel where the performance gain is even more prominent.
In order to generate proper weights for different beams, channel state information (CSI) is needed at the transmitter side. For a time division duplex (TDD) system, when the user speed is below a certain threshold, channel impulse responses for uplink (UL) and downlink (DL) may be considered as the same or having a difference that can be neglected. In such a case, channel reciprocity between UL and DL exists. A reciprocity based multi-antenna solution is the most forward-looking multi-antenna technique for a TDD system, and has the highest performance potential for dedicated data transmission and reception. This solution relies on the strictest, so-called “coherent”, form of reciprocity, which is achievable only in the TDD system where the receiving (RX) and transmitting (TX) channels are considered as the same within a coherent time interval. By utilizing channel reciprocity, instantaneous channel information can be obtained based on uplink measurements and then used for both uplink and downlink beam-forming, thereby enabling full exploitation of the angular spread.
Reference signals can be transmitted to facilitate channel estimation/measurement at receiver side. In a Long Term Evolution (LTE) system developed by the Third Generation Partnership Project (3GPP), a Sounding reference signal (SRS) is used to enable uplink channel measurement. In LTE, the SRS transmission depends on a configuration set by a signaling message (e.g., System Information Block 2 (SIB2), Radio Resource Control (RRC) Connection Setup, RRC Connection Reconfiguration etc.). A terminal device may transmit the SRS every two subframes at the most and every 32 frames (i.e., 320 subframes) at the least. A 10 bit signaling parameter “srs-ConfigIndex” informs the terminal device of the periodicity of SRS transmission and a SRS transmission period can be one of 2, 5, 10, 20, 40, 80, 160 and 320 ms. Furthermore, there is also an option for the terminal device not to transmit the SRS at all. More details of the SRS can be found in 3GPP TS 36.211, section 5.5.3 “Sounding reference signal”, for example.
The next generation wireless communication system (also referred to as the fifth generation (5G), or NR) is being studied, and a reciprocity reference signal (RRS) similar to the SRS has been proposed for use in the NR system to extract the CSI for a transmitter (CSI-T) and to generate the DL beamforming weights accordingly.
Carrier aggregation (CA) is another technique exploited in a wireless communication system for boosting system capacity and data rate. Generally, a higher data rate requires more spectrum bandwidth and higher spectrum efficiency. With CA, the available spectrum can be located across several frequency bands, and by combining different carriers, both the peak data rate and total throughput can be maximized. Another advantage of CA is that load-balancing across frequencies and systems can be easily performed when there is congestion in one frequency band, while unused capacity is available in another frequency band. Carrier aggregation also enables a combination and efficient utilization of a plurality of non-contiguous or narrow (e.g., 5 MHz or less) channel bandwidths.
In an LTE-Advanced (also referred to as LTE-A) system, CA has been adopted in order to increase the bandwidth, and thereby increasing the data rate. In order to keep backward compatibility with LTE Release 8 (also referred to as R8) and Release 9 (also referred to as R9) user equipments (UEs), the carrier aggregation in LTE-A is based on R8/R9 defined carriers. There may be a few different carrier aggregation scenarios. For example, intra-band contiguous carrier aggregation is a straightforward way to arrange carrier aggregation, where contiguous component carriers within the same operating frequency band (as defined for LTE) are aggregated. This might not always be possible, however, due to operator's frequency allocation scenarios. Non-contiguous carrier aggregation is another option.
Currently, for a wireless communication system with CA, a UE usually transmits uplink signals only on a primary carrier (also referred to as Primary cell, Pcell, or Pcell carrier hereafter) and receives downlink signals on selected carriers which may include the primary carrier and one or more secondary carriers (also referred to as Secondary cell, Scell, or Scell carrier hereafter) to boost the downlink capacity. However, in conventional CA approaches, system performance might be degraded. For example, for secondary carriers without uplink transmission from the UE, the corresponding channel state information (CSI) can hardly be obtained by the network side.